A multiphase plate tectonic history of the southeast continental margin of Oman

Abstract

Abstract The major basins of the western Indian Ocean formed during the Late Jurassic separation of India-Madagascar-Antarctica from Africa-Arabia. From north to south these are the Owen, North Somali, West Somali, and Mozambique Basins. Mesozoic magnetic anomalies confirm that these latter two regions have remained tectonically unchanged; by contrast, anomalies have yet to be identified in the Owen and North Somali Basins, leaving open the possibility that one or both have been overprinted by subsequent spreading events. Nonetheless, many researchers have chosen to assume that the basement within the Owen Basin is an unaltered fragment of Jurassic to Early Cretaceous oceanic crust, and that SE Oman has been a passive margin since its rifting from India. Marine geophysical data acquired aboard R/V Robert D. Conrad during 1986 provides new information that suggests a more complex regional plate history must be developed. Seismic reflection profiles collected across the Owen Basin reveal structure that is not consistent with a Jurassic age. First, we observe that basement in the Owen Basin is more than 1 km shallower than should be expected from thermal subsidence since Jurassic time. Second, the age of sediments in the Owen Basin can be estimated by tracing them seismically to DSDP Site 224 on the Owen Ridge. Both of these procedures argue for the Owen Basin being no older than Late Cretaceous. Seismic profiles along the SE Oman Margin near 18°N show complex deformational features that are incompatible with a simple passive margin history. A thick sedimentary basin bounded by basement-involved faults was mapped beneath the continental slope. Offsets reach to very near the seafloor, suggesting they have been active in the recent past. We acquired numerous crossings of a basement feature 75 km SW of Ra’s Madrakah and along the strike of ophiolites exposed there and at Masirah Island. The magnetic, seismic, and structural characteristics of this feature are all consistent with its being a buried, along-strike extension of this same slice of oceanic crust. To reconcile these observations with well-established facts concerning the opening history of the NW Indian Ocean, we propose the following scenario. First, a proto-Owen Basin formed in Jurassic time when India-Madagascar-Antarctica separated from Africa-Arabia. However, following the Late Cretaceous separation of India from Madagascar, the new spreading ridge was offset along the SE Oman margin, not at the Owen Ridge as is assumed by many. During this stage of tectonic development, the proto-Owen Basin crust was carried northeastward towards eventual collision and subduction with Asia. The result was a strike-slip Oman margin adjacent to a Late Cretaceous Owen Basin. Ophiolites now found along the SE coast of Oman are fragments of this Late Cretaceous Owen Basin crust, and were probably emplaced as a result of transpressional motion along this plate boundary. This arrangement persisted until yet another spreading centre opened the Gulf of Aden in Neogene time. The latter reorganization of plate geometries led to major compressive stresses across the Owen Basin that resulted in elevation of the Owen Ridge (readily seen by way of stratal relationships in profiles tied to DSDP Site 224), and reactivation of Late Cretaceous faults along the Oman Margin. We anticipate that verification of this multiple-phase history will emerge from future comparisons between post-Late Cretaceous tectonism within SE Oman and our marine seismic and drilling data.